If I were you, I would go back to the store for a couple reasons. Because it is so difficult for you in several ways, I think there is a lot of opportunity to "practice" working on those personal traits that worry you so when you graduate and get a different kind of job (what are you majoring in?) you will have experience working on your confidence and doing tasks that are scary for you and seeing you can succeed.

I would use the fact that the job market is tough and they don't have to hire you back, they could hire someone else if you were unsatisfactory, to help myself feel better about myself. This company does like you, despite what you feel because they keep hiring you back! There are other students like you out there looking for summer jobs but they have hired you consistently.

You cannot learn to feel better about yourself if it is all sunshine and roses and a perfect, lovely job (which does not exist, all jobs have difficult/unpleasant tasks and people and problems) because feeling better about yourself can not come from outside you, you have to build it through getting hired back repeatedly and seeing that that's a good thing and being proud of yourself :-)

Everyone makes mistakes and forgets stuff and has to learn new things for the first time in a fumbling/non know-what-they're-doing fashion! You can't know what you don't know! That's not a bad thing, criticism, or anything to be ashamed or afraid of. Learn all you can about the company and the people, etc. while you have the opportunity. Depending on your major, you might want to work at corporate in a higher level job when you graduate and you'd be "ahead" of the competition because of your time spent working in their store. Even if you don't work for this company, companies are all the "same" in many aspects; they all have to sell something and do accounting, be administered/run, have customer service, etc. That's "business". What you learn is up to you.